Closed-loop power control is a technique widely used in wireless communication systems such as universal terrestrial radio access (UTRA) frequency division duplex (FDD), UTRA time division duplex (TDD) 3.84/1.28 Mcps, code division multiple access (CDMA) one, or the like. The transmission power of a WTRU or base station is adjusted periodically according to feedback information sent in the opposite link by a receiver in the base station or WTRU.
For example, consider closed-loop power control in a DL which controls the power transmitted by a base station for a specific WTRU. The WTRU receives the signal from the base station during a certain period of time, (e.g., a time slot or a frame), and determines whether or not the transmission power of the base station needs to be adjusted up or down using a particular quality criterion, such as the signal-to-noise-plus-interference ratio. At the time of the next transmission from the WTRU to the base station (UL), the WTRU sends, along with other UL data, a TPC command containing the relevant information for the base station to adjust its transmission power at the subsequent DL transmission.
In many systems, the TPC command contains only one information bit indicating whether the power should be increased or decreased by a pre-determined amount, or step size. The accuracy of the power adjustment may be improved by using more than one information bit per TPC command, (allowing multiple step sizes), or by increasing the frequency of the TPC commands. The disadvantage of doing this is that the amount of transmitted UL data needs to be reduced to create room for this additional TPC information. Thus, there is a trade-off between power control accuracy in one direction and the data rate in the opposite direction.
In UTRA TDD, (3.84 Mcps and 1.28 Mcps), DL power control for dedicated physical channels (DPCHs) and physical DL shared channels (PDSCHs) is closed-loop and works in the manner described in the above paragraphs. In addition, in UTRA TDD, (1.28 Mcps option only), UL power control is also closed-loop for DPCHs and physical uplink shared channels (PUSCHs).
The following standard parameters apply to UTRA TDD with respect to closed-loop power control. Each TPC command (up or down) consists of one information bit. The TPC step size (up or down) can be 1 dB, 2 dB or 3 dB and is determined at radio link setup. For DL power control, a DL channel is associated to at least one UL channel that provides one or more TPC commands. This DL channel may be a multiplexed channel of several transport channels (TrCHs), where each TrCH can carry a different communication service. This multiplexed channel is referred to as a CCTrCH.
Typically, there is a single UL CCTrCH mapped to each DL CCTrCH, and there is one TPC command every frame of 10 ms (for 3.84 Mcps option) or sub-frame of 5 ms (for 1.28 Mcps option). Conversely, for UL power control, (for the 1.28 Mcps option only), a UL CCTrCH is associated to a DL CCTrCH that provides the TPC commands. Typically, there is one TPC command every sub-frame of 5 ms. There may be more than one TPC command if the UL CCTrCH occupies more than one time slot (one TPC command per time slot every sub-frame).
A CCTrCH subject to power control, (a DL CCTrCH for DL power control, a UL CCTrCH for UL power control), is referred to as the power-controlled CCTrCH. A feedback CCTrCH is the CCTrCH to which the power-controlled CCTrCH is associated, and which provides it with the TPC commands, (a UL CCTrCH for DL power control, a DL CCTrCH for UL power control). The base station or WTRU transmitting the power-controlled CCTrCH also receives the feedback CCTrCH, while the base station or WTRU receiving the power-controlled CCTrCH also transmits the feedback CCTrCH.
DTX is employed in UTRA TDD systems, (1.28 Mcps and 3.84 Mcps), on a CCTrCH basis when there is no data to transmit for this CCTrCH. A CCTrCH supports part or all of the transmissions of a user. A user may use one or several CCTrCH's within a given timeslot. When DTX is activated for a CCTrCH, there will be no transmission on any physical channel supporting this CCTrCH, except for the first physical channel and only every special burst generation period (SBGP) frames (for uplink) or every special burst scheduling parameter (SBSP) frames (for DL), where SBGP or SBSP is configured at radio link setup. The use of DTX results in significant system and user performance benefits as less interference is generated in the system, and handset battery life may be conserved in the UL.
A problem occurs in UTRA TDD (3.84 Mcps and 1.28 Mcps) when DTX is used on a UL CCTrCH. This UL CCTrCH is the feedback, (i.e., the CCTrCH providing the TPC command), for a DL power-controlled CCTrCH. When this UL CCTrCH is in DTX, the TPC command is only transmitted when the special burst is transmitted, i.e., at every SBGP frame. As a result, the frequency transmission power updates are reduced dramatically and the result can be poor performance for the power-controlled CCTrCH. The same problem would occur in UTRA TDD (1.28 Mcps option only) when DTX is used on a DL CCTrCH and when this DL CCTrCH is the feedback CCTrCH for a UL CCTrCH. In this case, the UL performance would suffer. However, this performance deterioration could be mitigated if the TPC size were increased while the feedback (UL) CCTrCH is in DTX.
In a conventional wireless communication system, a single TPC step size is used regardless of the frequency at which the TPC commands arrive on the feedback CCTrCH. In case of DTX on the feedback CCTrCH, the TPC commands may arrive from 2 to 256 times less frequently than in a normal transmission (depending on the value of the SBGP or SBSP). Thus, the system designer or operator has three different options with respect to the configuration of a radio link when DTX can occur in the feedback CCTrCH.
A first option is to use a TPC step size optimized for normal transmission on the feedback CCTrCH, and experience poor performance in the power-controlled CCTrCH during the DTX transmission on the feedback CCTrCH. This is not acceptable if DTX happens often in the feedback CCTrCH.
A second option is to use a TPC step size optimized for DTX transmission on the feedback CCTrCH, and experience sub-optimal performance in the power-controlled CCTrCH during normal transmission on the feedback CCTrCH. Sub-optimal performance would result from using a larger-than-necessary TPC step size during normal transmission on the feedback CCTrCH, when TPC commands arrive frequently.
A third option is to reduce the special burst periodicity (SBGP or SBSP) to the minimum possible value so as to reduce as much as possible the impact on the performance of the power-controlled CCTrCH. This would result in wiping out any capacity or battery consumption benefit from the use of DTX in the feedback CCTrCH.
A fourth option is to use more than one information bit in the TPC command, enabling the node transmitting the feedback CCTrCH to signal a larger step size when it is using DTX. However, this has the disadvantage of reducing the capacity of the feedback CCTrCH as explained earlier.
None of these options is satisfactory, and would result in a loss of system performance. What is needed is enhanced performance in both normal and DTX cases for the feedback CCTrCH.